Observation optical system

Abstract

An observation optical system has a negative lens unit placed at the most object-side position; an annular prism placed on the image side of the negative lens, having a reflecting surface on the negative lens side; and an imaging lens unit arranged on the image side of the negative lens and the annular prism.

What is claimed is: 
     
       1. An observation optical system comprising:
 a negative lens, which is placed at a most object-side position; 
 an annular prism which is placed on at a more image-side position than the negative lens, the annular prism having a reflecting surface that is arranged on a negative-lens side of the annular prism and that faces toward the image side position; and 
 an imaging lens unit arranged at a more image-side position than the negative lens and the annular prism, 
 wherein the negative lens and the imaging lens unit form a forward observation optical system and the annular prism and the imaging lens unit form a backward observation optical system, and an image produced by the backward observation optical system and an image produced by the forward observation optical system are formed at different positions on an imaging plane, and 
 wherein the forward observation optical system and the backward observation optical system have an aperture stop common to each other. 
 
     
     
       2. The observation optical system according to  claim 1 , wherein the negative lens and the annular prism are cemented. 
     
     
       3. The observation optical system according to  claim 2 , used in an endoscope. 
     
     
       4. The observation optical system according to  claim 1 , wherein a region that forms a blind spot with respect to both the negative lens and the annular prism is generated around a lateral portion of the negative lens and the annular prism, and an illumination means is placed in the blind spot. 
     
     
       5. The observation optical system according to  claim 4 , used in an endoscope. 
     
     
       6. The observation optical system according to  claim 1 , used in an endoscope. 
     
     
       7. The observation optical system according to  claim 1 , wherein the annular prism is hollowed so that a bundle of rays passing through the negative lens is not incident on the annular prism. 
     
     
       8. The observation optical system according to  claim 1 , wherein the negative lens and the annular prism are constructed and arranged so that a bundle of rays passing through the negative lens is directly incident on the imaging lens unit. 
     
     
       9. The observation optical system according to  claim 1 , wherein there is no optical member other than the annular prism arranged between the negative lens and the imaging lens unit. 
     
     
       10. The observation optical system according to  claim 1 , wherein the annular prism is arranged to reflect a bundle of rays incident thereon only once. 
     
     
       11. The observation optical system according to  claim 1 , wherein the annular prism further comprises a first refracting surface and a second refracting surface; and the reflecting surface, the first refracting surface, and the second refracting surface are configured in the annular prism so that a bundle of rays entering the annular prism through the first refracting surface is then reflected at the reflecting surface and emerges from the second refracting surface. 
     
     
       12. An observation optical system comprising:
 a negative lens, which is placed at a most object-side position; 
 an annular prism which is placed at a more image-side position than the negative lens, the annular prism having a reflecting surface that is arranged on a negative-lens side of the annular prism and that faces toward the image side position; and 
 an imaging lens unit arranged at a more image-side position than the negative lens and the annular prism, 
 wherein the negative lens and the imaging lens unit form a forward observation optical system and the annular prism and the imaging lens unit form a backward observation optical system, and an image produced by the backward observation optical system and an image produced by the forward observation optical system are formed at different positions on an imaging plane, and 
 wherein the observation optical system is used in an endoscope.